Generally, sound that is heard by human beings is a wave. The sound wave that is transmitted to the ears of the human beings vibrates the air molecules within the ears. The movement of the molecules, in turn, vibrates the tympanic membrane, that is, eardrum so that the human beings can hear the sound.
The sound wave enters the external ears of the human beings and is transmitted to the tympanic membrane. The vibration of the tympanic membrane generated by the sound wave is transmitted to the cells of the internal ears, so that the human beings can hear the sound.
The audible frequency in which the human beings can hear the sound is within the range of 20 Hz to 20,000 Hz. The best audible range in the audible frequency range, that is, so called ‘communication voice range’ is 125 Hz to 8,000 Hz. Commonly, the human beings use the range of 500 Hz to 8,000 Hz for their communication.
Most of the human beings can hear the sound in the range of 20 Hz to 20,000 Hz. However, some of the human beings having auditory disorders such as have disorders of external ears or middle ears, presbycusis which is a hearing loss that gradually occurs as they grow older, and hearing loss as an occupational disease can not hear the sound in the only specific frequency range or over the all frequency range.
A bone conduction telephone and a hearing aid vibrator by using a piezoelectric device which is used for such people having hearing loss are disclosed in the Korean utility model registration No 221534.
The bone conduction telephone and the hearing aid vibrator by using the piezoelectric device according to the utility model in the Korean utility model registration No 221534 comprises the piezoelectric device 100 which generates a physical vibration by an electric voice signal being applied, vibrating plates 110 disposed at both end side of the piezoelectric device 100 which apply the electrical signal to he piezoelectric device 100 through a conducting wire 130, an electrode plate 120, a front contacting portion 140 connected to the vibrating plates 110 which transmits the vibration to the exterior, and a cover which supports and protects the piezoelectric device 100. In the utility model, the piezoelectric device 100 generates a vibration with piezoelectric effect. The generated vibration has a voice frequency corresponding to the electrical voice signal which is applied through the vibrating plates 110 and the electrode plate 120. The generated vibration is transmitted to the exterior as a vibration signal.
However, bone conduction telephone and the hearing aid vibrator by using the piezoelectric device according to the utility model are too theoretical to be implemented. The utility model has so many problems for its commercial implementation. Since the piezoelectric device has high impedance, the output sound pressure is too low and the miniaturization of the products is too difficult.
In order to solve the problems, a bone conduction hearing aid apparatus and a bone conduction telephone that directly vibrate skull bone have bee proposed and developed. In the bone hearing aid apparatus and the bone conduction telephone, the vibration stimulates the auditory nerve and thus the sound can be heard.
The bone conduction speaker which directly vibrates the skull bone and the vibration stimulates the auditory nerve for hearing the sound is disclosed in the Korean patent registration No. 310891 which is registered on Sep. 21, 2001.
The bone conduction speaker disclosed in the Korean patent registration No. 310891 comprises a frame having side wall and lower wall which defines a certain space; a protector of which inner side peripheral portion is combined to upper surface of the side wall and of which center has an opening; a damper for reducing intensities vibration and sound by performing stabilized vibration which is disposed between the upper end of the side wall and the lower surface of the inner side peripheral portion; a main body for supporting the inner side end of the damper which is disposed within the space; an extending portion which is incorporated into the main body and extended downwardly; an yoke for focusing magnetic flux; a magnet which is disposed at the interior of the extending portion and of which upper surface is closely contacted to the lower surface of the main body; a vibration contacting portion which is externally exposed through the opening to directly contacted to head of human beings; a fixing means for fixing the protector, the yoke, the magnet, and the upper plate as one body; and a voice coil which is disposed in the clearance between the upper plate, both ends of the magnet, and the inner surface of the extending portion and of which lower end is fixed to the lower wall.
However, the conventional bone conduction speaker has a problem that high driving voltage and high power consumption are necessary to vibrate the yoke, the magnet, and the upper plate since the yoke, the magnet, and the upper plate serve as parts for generating the magnetic field as well as parts of vibrating upwardly and downwardly.
In addition, in the conventional bone conduction speaker, high strength of the magnetic field is necessary and large sizes of the yoke, the magnet, and the upper plate which are above a specific size are necessary to vibrate the yoke, the magnet, and the upper plate since the yoke, the magnet, and the upper plate serve as parts for generating the magnetic field as well as parts of vibrating upwardly and downwardly.
Therefore, the conventional bone conduction speaker has another problem that it is impossible to miniaturized and be employed to a subminiature mobile phone, a subminiature headset, a hearing aid apparatus, and so on.
Furthermore, in the aforementioned bone conduction speaker, the voice coil is fixed on the base plate and the mastoid, the yoke, the magnet, and the upper plate are vibrated by the magnetic field which is generated by the voice coil. Therefore, the conventional bone conduction speaker has still another problem that it is impossible to transmit the vibration of the mastoid to the user accurately since the mastoid which is contacted to the bone skull of the user is difficult to vibrate upwardly and downwardly due to heavy weights of the mastoid, the yoke, the magnet, and the upper plate.
In addition, the conventional bone conduction speaker has further still another problems that it may be easily being out of order due to any infinitesimal external impacts and its reliability may be being deteriorated since the internal parts such as the yoke and the magnet for generating the magnetic field are disposed at the upper portion of the bone conduction speaker.